Engineering the magnetic properties of hybrid organicferromagnetic interfaces by molecular chemical
functionalization
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Atodiresei, Nicolae et al. “Engineering the Magnetic Properties of
Hybrid Organic-ferromagnetic Interfaces by Molecular Chemical
Functionalization.” Physical Review B 84.17 (2011): Web. 18
May 2012. © 2011 American Physical Society
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http://dx.doi.org/10.1103/PhysRevB.84.172402
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American Physical Society
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PHYSICAL REVIEW B 84, 172402 (2011)
Engineering the magnetic properties of hybrid organic-ferromagnetic interfaces
by molecular chemical functionalization
Nicolae Atodiresei,1,* Vasile Caciuc,1 Predrag Lazić,2 and Stefan Blügel1
1
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, D-52425 Jülich, Germany
2
Massachusetts Institute of Technology, Cambridge, 02139 Massachusetts, USA.
(Received 4 April 2011; published 10 November 2011)
We have performed systematic first-principles calculations to tailor the magnetic properties at a hybrid organicferromagnetic interface by adsorbing organic molecules containing π (pz ) electrons onto a magnetic substrate. For
such hybrid systems, magnetic properties such as molecular magnetic moments and their spatial orientation can
be specifically tuned by substituting the H atoms with more electronegative atoms such as Cl and F. This chemical
functionalization process surprisingly reveals the importance of the spin-orbit coupling present at the magnetic
surface–molecule interface. As a key result, our simulations indicate a direct connection between substituent
electronegativity and these magnetic properties which can be exploited to design more efficient organic spintronic
devices.
DOI: 10.1103/PhysRevB.84.172402
PACS number(s): 73.20.−r, 68.43.Bc, 71.15.Mb
Progress in developing cutting-edge molecular electronic
devices1–3 which make use of both the electron’s spin degree
of freedom and organic molecules relies on a clear theoretical
understanding of the complex phenomena present at the
electrode-molecule interfaces. In particular, of significant
technological interest are the new functionalities of organic
molecules when adsorbed on a magnetic substrate. Nowadays,
a large effort is dedicated not only to understanding the
molecule–magnetic surface interfaces but also to engineering
and precisely controlling their spin-sensitive tunneling channels built between the molecules and the magnetic substrate.4–8
In recent combined ab initio and experimental studies, we
demonstrated the universal applicability of a theory-driven
concept10 that indicates the importance of interface hybrid
molecular-metal states rather than the intrinsic molecular
spin11 on defining the spin polarization present at hybrid
organic-magnetic interfaces. Besides creating a well-defined
hybrid organic-ferromagnetic interface that acts as an effective
source of spin-polarized electrons, several essential issues
in spintronics remain to be resolved: (1) how to induce a
magnetic moment in an organic layer, (2) how to control
the size of the induced magnetic moments, and (3) how to
control their orientation at the molecular sites. Therefore in
this Brief Report we will generalize our conceptual study10
to hybrid magnetic organic-ferromagnetic interfaces to tune
their magnetic properties by using specific molecular chemical
substituents. The aim of this chemical functionalization is to
(1) precisely manipulate the local spin polarization present
at a magnetic molecule-ferromagnetic substrate interface and
more importantly to determine (2) how to induce and enhance
a magnetic moment in the adsorbed organic molecules and
(3) how to locally stabilize the magnetization direction of
a ferromagnetic surface by the absorption of specifically
functionalized organic molecules. In the later case our calculations clearly emphasize the crucial role played by the
light elements building the organic molecules on the the
spin-orbit coupling (SOC) occurring at the hybrid organicferromagnetic interface. Importantly, this SOC can be manipulated using a different electronegativity of specific substituents
(Cl and F).
1098-0121/2011/84(17)/172402(4)
Our first-principles simulations9 are performed on prototype organic molecules adsorbed on a ferromagnetic 2
monolayers (ML) Fe/W(110) surface, which is a wellestablished system with out-of-plane magnetization widely
used in spin-polarized scanning-tunneling microscopy (SPSTM) experiments.12 Similar to our previous study,10 in a first
step we selected organic molecules containing π (pz )-electron
systems such as benzene (C6 H6 )13–15 and cyclopentadienyl
radical (C5 H5 )16 since they correspond to classes of organic
molecules with significantly different chemical reactivities.17
In a second step, we chemically functionalized the C6 H6
and C5 H5 molecules by replacing all H atoms through
electronegative atoms as F and Cl to obtain a similar π (pz )electron systems. However, owing to the significantly higher
electronegativity of F and Cl atoms than that of the H atom,
when F and Cl atoms form σ bonds with a carbon atom of the
aromatic ring, they exert a strong inductive electron withdrawal
effect (with charge density being displaced from C toward F
or Cl) and, as a consequence, π conjugation is decreased.17
The spin-polarized ab initio studies are carried out in the
framework of density functional theory (DFT) by employing
the generalized gradient approximation (PBE)18 in a projector
augmented plane-wave formulation19 as implemented in the
VASP code.20,21 The molecule-Fe/W(110) system is modeled
within the supercell approach [a p(5×3) in-plane surface unit
cell] and contains 11 atomic layers (9 W and 2 Fe). By
using a plane-wave energy cutoff of 600 eV in our ab initio
calculations, the uppermost two Fe layers and the molecule
atoms are allowed to relax until the atomic forces are lower
than 0.001 eV/Å.22 As depicted in Fig. 1, the C6 H6 molecule
adsorbs with two C atoms on top of two neighboring Fe atoms
along the [001] direction while the other four C atoms sit in
bridge positions between Fe atoms of adjacent [001] rows.
With one fewer C than C6 H6 , C5 H5 binds to the surface with
a C atom and a C–C bond on top of two neighboring Fe atoms
of the [001] row. The fluorinated and chlorinated molecules
[C6 X6 and C5 X5 (X = F, Cl)] adsorb in a similar geometry
as the C6 H6 and C5 H5 but rotated by 90◦ (see Fig. 1). Note
that, compared to planar molecular geometry of the isolated
gas phase, after adsorption all molecules have a nonplanar
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©2011 American Physical Society
BRIEF REPORTS
PHYSICAL REVIEW B 84, 172402 (2011)
FIG. 1. (Color online) The spin-resolved projected local density of states of the C6 X6 and C5 X5 (X = H, F, Cl) molecules adsorbed on the
2ML Fe/W(110) are significantly broadened due to an effective hybridization between the molecule and the Fe surface. All adsorbed molecules
present a general characteristic: energy-dependent spin polarization, i.e., in a given energy interval the number of spin-up and spin-down
electrons is unbalanced and each molecule has a net magnetic moment. Note that the states with large weights crossing the Fermi level are
situated in the spin-down channel at the metal site (see Fig. 2) while, at the molecular site, above the Fermi level the states with large weights
are in the spin-up channel. Therefore, at the molecular site, above the Fermi level, an inversion of the spin polarization occurs with respect
to the ferromagnetic surface. For the energy intervals situated just below the Fermi level, the molecules which contain less electronegative
atoms such as H and Cl [C6 X6 and C5 X5 (X = H, Cl)] show also the inversion of the spin polarization while for the molecules containing very
electronegative atoms such as F (C6 F6 and C5 F5 ) the spin polarization is preserved like the one of the clean iron surface (see also Fig. 3).
structure in which the H, F, and Cl atoms are situated above
the C atoms.
In contrast to our previous study,10 from the magnetic point
of view a very interesting feature is that upon adsorption on the
ferromagnetic surface the molecules with F and Cl substituents
[C6 X6 and C5 X5 (X = F, Cl)] have a sizable magnetic
moment24 (see Table I) which is antiferromagnetically oriented
relative to the iron substrate. In contrast, the hydrogenated
molecules (C6 H6 and C5 H5 ) are characterized by small
magnetic moments that can practically be neglected. Even
more interesting is that the magnetic moment of the adsorbed
molecule correlates with the electron affinity of the substituent.
The molecules containing F, a much more electronegative
atom compared to Cl,17 have a much larger magnetic moment
with respect to the chlorinated molecules while the molecules
having a less electronegative atom such as H are practically nonmagnetic. Due to the molecule-substrate interaction
the magnetic moments of the Fe atoms situated just below the
organic molecule are decreased compared to the ones of the
clean Fe surface (see Table I). The decrease of the magnetic
moments of these Fe atoms can also be correlated with the
increase of the electronegativity of the substituents; i.e., the
TABLE I. Magnetic moments (MM) of the adsorbed molecules and the Fe atoms below an C atom (Fe1 ) or a C=C double bond (Fe2 ) and
magnetic anisotropy energies (MAE) for the adsorbed molecule-substrate systems given in meV per unit cell [corresponding to 60 Fe atoms of
a double layer of 2ML Fe/W(110)]. With increasing electronegativity of the substituent from H to Cl to F the MM of the adsorbed molecule
increases while the MM of the Fe atoms below the molecule decrease. For the most stable magnetic configuration the magnetization directions
is out of the plane (perpendicular to the surface, i.e., aligned with the [110] direction). The positive numbers in the last two columns show with
how much the total energy of the molecule-substrate system increases when the magnetization is in plane aligned along the [11̄0] and [001]
directions.
Magnetic moments (μB )
Molecule
C6 H6
C 6 F6
C6 Cl6
C5 H5
C 5 F5
C5 Cl5
−0.076
−0.278
−0.164
−0.031
−0.262
−0.127
2ML FeW(110)
MAE (meV)
(C)
Fe1
(C=C)
Fe2
+2.515
+1.446
+1.755
+2.471
+1.776
+1.855
+2.655
+2.270
+2.229
+2.540
+2.198
+2.228
2.821
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[110]
per unit cell
[11̄0]
[001]
0.000
0.000
0.000
0.000
0.000
0.000
0.000
21.226
29.780
26.875
20.862
23.779
25.766
28.778
73.219
86.402
82.596
73.318
77.256
82.530
77.024
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PHYSICAL REVIEW B 84, 172402 (2011)
FIG. 2. (Color online) (a) The adsorption energies (Eads )23 show
that the C5 X5 reactive radicals interact more strongly with the Fe
substrate compared to the C6 X6 molecules (X = H, F, Cl). (b) The
spin-resolved projected local density of states of an Fe atom of the
clean surface (upper panel) and an Fe atom below the C atom of
C5 H5 (lower panel) indicate that the majority of electrons are in
the spin-up channel while the minority electrons are in spin-down
channel.
smallest magnetic moments correspond to the Fe atoms below
fluorinated molecules, followed by the chlorinated, and then
by the hydrogenated ones.
From the analysis of the spin-resolved projected local
density of states (PLDOS) of our molecule-surface systems
(Fig. 1) we can extract the general picture of the binding
mechanism for these hybrid molecular-magnetic substrate
systems. Upon adsorption a spin-dependent hybridization will
occur between the pz atomic type orbitals which originally
form the π -molecular orbitals and the d states of the Fe
atoms. This leads to the formation of new molecule-metal
hybrid states with bonding and antibonding character.10,11 As
a general feature of the spin-polarized PLDOS shown in Fig. 1,
in the spin-up channel the hybrid bonding states are situated
at low energies while the antibonding combinations of the
molecule-surface orbitals appear at much higher energies,
more exactly in an energy window situated around the Fermi
level. As a consequence, the hybrid antibonding states with
large weight around the Fermi level are in the spin-up channel
at the molecule site (see Fig. 1) while at clean metal sites
the states with large weight are in the spin-down channel [see
Fig. 2(b)].
Another interesting feature is that for the hydrogenated
molecules the hybrid antibonding orbitals are located mostly
below the Fermi level, while for the chlorinated and fluorinated
molecules these antibonding orbitals are emptied; i.e., they are
pushed above the Fermi level. This feature correlates with the
magnetic moment of the adsorbed molecules: the molecules
containing F atoms have the largest magnetic moments, which
is decreased in the case of chlorinated molecules, while the
molecules containing H have negligible magnetic moments.
As already shown for C5 H5 and C6 H6 ,10,11 the spinpolarized PLDOS of the chemisorbed functionalized fluorinated and chlorinated molecules also displays an energydependent spin polarization. This means that for a specific
energy interval below or above the Fermi level, the number
of spin-up and spin-down hybridized molecule-surface states
is unbalanced (see Fig. 1) and in this energy interval each
molecule has a net magnetic moment. As a consequence, above
FIG. 3. (Color online) The calculated spin polarization at 3.0
Å above the C6 H6 , C6 F6 , and C6 Cl6 molecules adsorbed on a
2ML Fe/W(110) (15.85 × 13.45 Å) surface plotted for occupied
([−0.3,0.0] eV) and unoccupied ([0.0, + 0.3] eV) energy intervals
around the Fermi level. The organic molecules containing H and Cl
atoms show above and below the Fermi level a high, locally varying
spin polarization ranging from attenuation to inversion with respect
to the ferromagnetic Fe film. In contrast, fluorinated molecules such
as C6 F6 preserve the spin polarization of the Fe surface for energy
intervals just below the Fermi level and show an inversion of the spin
polarization only for energy intervals above the Fermi level.
the Fermi level, for all studied molecules an inversion of the
spin polarization25 occurs at the molecular site with respect
to the ferromagnetic surface (Fig. 2). For energy intervals
below the Fermi level the same feature is present for molecules
containing H and Cl substituents. In contrast, this is not the case
for the molecules containing very electronegative atoms such
as F. Therefore, for energy intervals below the Fermi level, the
fluorinated molecules have the same spin polarization as the
iron substrate and show an inversion of the spin polarization
only for energy intervals above the Fermi level. These effects
are clearly seen in Fig. 3, which illustrates spin-polarization
maps at 3.0 Å above the molecule in the energy intervals below
([−0.3,0.0] eV) and above ([0.0, + 0.3] eV) the Fermi level.
Furthermore, we have included in our simulations the effect
of SOC by calculating several magnetic configurations with
a given direction of magnetization. The energy difference between these magnetic configurations represents the magnetic
anisotropy energy (MAE) and gives an estimate for the energy
barrier necessary to stabilize the magnetic moments against
quantum tunneling and thermal fluctuations.7 We considered
three possible surface-symmetry-determined directions of the
magnetization in the molecule-surface systems: out of plane
along the [110] direction (perpendicular to the surface) and
in plane along the [11̄0] and [001] directions (parallel to the
surface). The clean surface and all molecule-substrate systems
were relaxed by including spin-orbit coupling and without any
symmetry restrictions.
The most stable magnetic configuration of the clean surface
is obtained when the magnetization direction is pointing out of
the plane (along [110]) while when aligning the magnetization
direction in plane along [11̄0] ([001]) the total energy of the
system will increase with 0.480 (1.280) meV per Fe atom.26
By adsorbing organic molecules such as Cn Xn (n = 5,6;
X = H, Cl, F) on the 2ML Fe/W(110) surface the most
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PHYSICAL REVIEW B 84, 172402 (2011)
stable magnetic configuration is the same as that of the clean
surface; i.e., the magnetization direction is perpendicular to the
surface (see Table I). However, in the case of hydrogenated
molecules (C6 H6 and C5 H5 ) adsorbed on the iron surface,
the energy difference between the two most stable magnetic
configurations (magnetization pointing in the [110] and [11̄0]
directions) are decreased by a much larger amount with respect
to the clean surface compared to the functionalized chlorinated
and fluorinated molecules such as C6 Cl6 , C5 F5 , and C5 Cl5 .
By analyzing the adsorption energies Eads [see Fig. 2 (a)]
and the values of the MAE (see Table I) we can observe
that the energy difference between out-of-plane and in-plane
magnetic configurations decreases much more if the molecule
binds more strongly to the surface. Therefore, we conclude
that the out-of-plane magnetization present in the calculated
molecule-substrate systems is less stable against temperature
fluctuations compared to the clean ferromagnetic surface.
Surprisingly, this is not the case for the C6 F6 molecule, which,
after adsorption on the ferromagnetic surface, stabilizes more
the out-of-plane magnetization of the system compared to the
clean ferromagnetic surface. In particular, C6 F6 has the highest
magnetic moment and binds less strongly to the surface compared to all other molecules. This behavior can be correlated
with a specific structure of the spin-polarized PLDOS around
the Fermi level at the molecular site. More specifically, in the
case of the hydrogenated molecules the antibonding hybrid
molecule-surface combinations are situated below the Fermi
level while for the fluorinated molecules they are shifted above
the Fermi level.
To summarize, our first-principles calculations demonstrate
that, by employing an appropriate chemical functionalization
of organic molecules adsorbed on a ferromagnetic surface,
they can become magnetic and a fine tuning of their spinunbalanced electronic structure can be achieved. Even more
important is that we have shown that there is a direct
correspondence between the substituent’s electronegativity
and the size of the induced molecular magnetic moment. We
demonstrated that the spin-orbit coupling at the interface can
be manipulated by specific organic molecules such that the
adsorbed hydrogenated molecules destabilize more the out-ofplane magnetization of the ferromagnetic surface compared to
molecules containing more electronegative atoms such as Cl
and F, which could also enhance it. To be more specific, in the
case of fluorinated molecules the magnetic contrast detected
by an SP-STM tip with out-of-plane magnetization will be
increased compared to that of an isolated surface. In contrast,
for hydrogenated molecules the magnetic contrast detected
by the SP-STM tip with out-of-plane magnetization will be
decreased. Ultimately, the understanding gained in our study
will allow us to precisely engineer the magnetic properties
of the hybrid organic-ferromagnetic interfaces, which can be
further exploited to design more efficient spintronic devices5,6
based on organic molecules.
*
17
n.atodiresei@fz-juelich.de
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We also performed relaxations including the van der Waals (vdW)
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initio calculations and we found that the vdW interactions slightly
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23
Eads = Esystem − (Emolecule + Esurface ), where Esystem , Emolecule ,
and Esurface are the total energies of the molecule-surface
system, isolated gas phase molecule, and clean surface,
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24
Integrating the spin-resolved PLDOS up to the Fermi level yields
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The spin polarization is defined as (n↑ − n↓ )/(n↑ + n↓ ), where n↑
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